The present invention relates to electrolysis cells for the production of metal by the reduction of a metal halide in a molten bath comprising the metal halide dissolved in at least one molten halide of higher electrodecomposition potential than the metal halide. More particularly, this invention relates to a cell of this type which has an inner refractory lining and, located adjacent to and in abutment with the lining, a plurality of electrodes. These electrodes are disposed horizontally and arranged in at least one vertical stack in a superimposed, spaced relationship defining inter-electrode spaces between each pair of adjacent electrodes. During operation of a cell of this type, molten bath circulates through the inter-electrode spaces, and metal and halogen gas are produced therein. The present invention relates to an improvement in such cells for sealing of at least some of the joints of abutment between the electrodes and the adjacent refractory lining in order to avoid the deleterious consequences of physical shifts in the electrodes during operation of the cell and for enhancing the circulation of bath in the cell.
Electrolysis cells for the production of metal by the reduction of a metal halide in a molten halide bath have been known for many years. It is also known that bath circulation in such cells may be important, and that bath circulation patterns may be enhanced by directing the flow of halogen gas produced in the cell. A cell which is reportedly useful in connection with a process for the production of either zinc or lead by the reduction of the appropriate metal chloride, is described in U.S. Pat. No. 1,545,383 of Ashcroft. The Ashcroft cell includes an insulated refractory lining and a series of inclined graphite plates which function as bipolar electrodes. These electrodes are located adjacent to and in abutment with the refractory lining, and are arranged in a superimposed, spaced relationship defining inter-electrode spaces between each pair of adjacent electrodes. The cell is adapted to contain a molten bath of metallic chlorides, in which the electrodes are immersed. As electrolysis proceeds in this cell, metal and chlorine are produced in the inter-electrode spaces. Because of the inclination of the electrodes, the metal produced in the cell flows, under the influence of gravity, downwardly across the cathode surfaces of the electrodes and through holes in the electrodes to a metal-collecting zone in the bottom of the cell. At the same time, the chlorine produced in the cell flows, because of its buoyancy, upwardly across the inclined anode surfaces of the electrodes and through holes therein to a gas-collecting zone in the top of the cell. The anode and cathode surfaces of the electrodes of this cell may be corrugated to facilitate the flows of metal and chlorine in the cell. These flows reportedly induce circulation of the bath in the cell and thereby facilitate continuous electrolysis.
A bipolar electrolysis cell for the production of lead by the reduction of lead chloride in a molten halide bath is described in Bureau of Mines Report of Investigations No. 8166, entitled "Recovery of Lead From Lead Chloride by Fused-Salt Electrolysis". This cell includes an inner refractory lining and a plurality of graphite plates which function as bipolar electrodes. These electrodes are located adjacent to and in abutment with the refractory lining, and are arranged in a vertical stack in a superimposed, spaced relationship defining inter-electrode spaces between each pair of adjacent electrodes. The electrodes are inclined slightly from the horizontal and grooved on the anode and cathode surfaces to direct the flow of lead and chlorine produced during electrolysis to opposite sides of the cell. Gaps on both sides of the stack of electrodes allow lead to flow downwardly and chlorine to flow upwardly on opposite sides of the stack.
A bipolar electrolysis cell which is particularly adapted for the production of aluminum by the electrolytic reduction of aluminum chloride in a molten halide bath is described in U.S. Pat. No. 3,893,899 of Dell et al. This cell includes an anode, at least one intermediate bipolar electrode and a cathode in a superimposed, spaced relationship defining inter-electrode spaces therebetween. These electrodes, unlike the electrodes in the Ashcroft cell, are preferably disposed horizontally within a vertical stack. Along one side of the stack of electrodes is located a bath-supply passage, which is in fluid communication with each inter-electrode space within the stack. Along the opposite side of the stack is a gas-lift passage, which is also in fluid communication with each inter-electrode space. As electrolysis proceeds in this cell, chlorine is produced on the anode surfaces of the electrodes, and metal is produced on the cathode surfaces. The chlorine is conducted through the inter-electrode spaces toward and into the gas-lift passage. This flow of chlorine induces a flow of molten bath into and out of each inter-electrode space, upwardly in the gas-lift passage, across the stack of electrodes and downwardly through the bath-supply passage. The flow of bath entrains metal produced on each cathode surface and carries it through and out of each inter-electrode space. The metal which is entrained by the flow of bath is carried into the gas-lift passage, where (except for fine droplets uncoalesced) it descends under the influence of gravity, in a direction opposite to that of the rising chlorine and bath, to the bottom of the cell.
In operating an electrolysis cell which includes an inner refractory lining and a plurality of electrodes located adjacent to and in abutment with the lining, and in which bath circulation is assisted by directing the flow of halogen gas produced therein, it has been found that inefficiencies in the bath circulation may arise from leakage of halogen gas and bath through the joints of abutment between the electrodes and the adjacent refractory lining. It is also known that electrodes in electrolysis cells may be subject to physical shifting within the cell during the operation thereof, even though such electrodes are located adjacent to and in abutment with an inner refractory lining. This shifting may result from mechanical stresses caused by thermal expansion or chemical reaction, or from buoyancy effects due to the relatively close densities of certain types of electrodes, especially carbonaceous electrodes, and the molten electrolyte bath in the cell. This shifting of electrodes may damage the electrodes, and it may produce gaps in the joints between the electrodes and the adjacent refractory lining. The appearance of these gaps may lead to penetration therethrough of metal and to deviations from the desired flow of halogen gas produced by the electrolysis process. This could result in re-halogenation of the metal and, in the case of carbonaceous electrodes, in the combination of the metal with the carbon of the electrodes on the surfaces thereof. Thus, halides and carbides of the metal may build up a sludge-like formation on the electrode surfaces, which would interfere with the efficient operation of the cell by reducing the anode-cathode spacing. Continued accumulation of sludge on the electrode surfaces could produce electrical short circuits, thus further impairing electrolysis.
The problem of shifting electrode blocks in electrolytic cells is discussed in U.S. Pat. No. 3,764,509 of Etzel et al. This reference discloses a means for minimizing the effect of mechanical stresses on the carbonaceous cathode in an electrolysis cell used for the production of aluminum. According to this reference, buckling or bulging of such a cathode due to mechanical stresses encountered in the operation of the cell can be minimized by providing the cathode in the form of a set of carbonaceous blocks, each block possessing four lateral surfaces, at least an opposed pair of which are inclined at different angles to the vertical. Thus, the assembled cathode obtains, by virtue of the shapes of its component blocks, a mutual wedging of the blocks against upwardly acting forces.
Another solution to the problem of the shifting of electrodes during operation of an electrolysis cell is described in U.S. Pat. No. 4,290,874 of McMonigle et al. According to this reference, an electrolysis cell may be provided with carbon felt gaskets located in the joints of abutment between electrodes or electrode elements and the adjacent refractory lining, in order to minimize or eliminate physical shifting of the electrodes during operation of the cell. These gaskets are provided in the form of sized pieces of a fabric of matted carbon fibers, which may be placed in notches that have been cut in the abutting electrodes or electrode elements at the joints to be sealed. Although these gaskets seal the joints of abutment between electrodes and the adjacent refractory lining against leakage of halogen gas and bath therethrough, they do not otherwise assist in the establishment and maintenance of the desired bath circulation.